Before I answer your question: 40 years is a very long time. Your ointment should have an expiration date. You should not use it past that date. Not just that microbiological quality can't be guaranteed after that much time (not even with phenol which might have preserved it to a point, but not for 40 years), but also the chemical composition might have changed - both the active substances and the excipients might have underwent various chemical reactions - and no one can say with any certainty what you've got in there now.
Now, onto your question: How important is the amount of active ingredient in an ointment?
It depends on the ingredient. The concentration is important because both safety and efficacy/effectiveness depend on it. Some ingredients have a very wide range of concentrations in which they are proven to be both safe and effective; others have a very narrow range of concentrations in which they should/(are recommended to) be used; some are somewhere in between. You are asking about an ointment with lesser concentrations of active substance than the one you previously used, so the efficacy of the 'new' ointment is what you are concerned with.
Phenol is used as an antiseptic in your ointments. The efficacy of antiseptics depends on many factors including:
- acidity (pH value) of the solution/medium/preparation
- duration of exposure
- the type(s) and number of microorganisms present
- presence of organic matter (especially in forming a biofilm)
So, the concentration is important, but there are other factors to be taken into account.
According to Martindale, The Complete Drug Reference (34th edition), the Phenol monograph:
Aqueous solutions up to 1% are bacteriostatic while stronger solutions are bactericidal.
Both ointments you used contain phenol in bacteriostatic (inhibits growth of bacteria) and not bacteriocidal (kills bacteria) concentrations.
There is various research on a MIC (minimal inhibitory concentration) of phenol:
According to Antiseptics and Disinfectants: Activity, Action, and Resistance G. McDonnell, A. D. Russell
Pulvertaft and Lumb (386) demonstrated that low concentrations of phenols (0.032%, 320 μg/ml) and other (nonphenolic) agents lysed rapidly growing cultures of E. coli, staphylococci, and streptococci
From the same source (based on references 226 and 440 provided in that research paper):
the MIC of phenol against S. aureus, E. coli and P. aeruginosa is 2,000 μg/ml (which is 0.2%)
In Protection of bacteria against toxicity of phenol by immobilization in calcium alginate Heribert Keweloh, Hermann-Josef Heipieper, Hans-Jürgen Rehm have used solutions of phenol in both 1 g/l (0.1%) and 2 g/l (0.2%) concentrations and got inhibition of growth of some bacteria (although they were testing something else, and I can't access the whole article).
Conclusion: concentration of phenol in your ointment is close to or over the ones I found in these references, and should be sufficient to exibit bacteriostatic activity if other conditions (such as pH, lipophilicity/hydrophilicity of the preparation etc) are favourable.
Zinc oxide is mildly astringent and is used topically as a soothing and protective application in eczema and slight excoriations, in wounds, and for haemorrhoids (Martindale).
It is used in concetrations of up to 50% (Deutscher Arzneimittel Codex – DAC), but technical literature doesn't specify a minimal concentration at which it should be used.
Perhaps I could just use more each time?
You could, but you shouldn't. Follow the instructions included in the patient information leaflet. (Using more most likely wouldn't enhance the efficacy anyway).
In the end if a product is marketed as a medicine/drug in your country, it has to be submitted to rigorous procedures before it is approved, i.e. authorised for sale.
Before drug products are authorized for sale in Canada, Health Canada reviews them to assess their safety, efficacy and quality. Drug products include prescription and non-prescription pharmaceuticals, disinfectants and sanitizers with disinfectant claims.
Prior to being given market authorization, a manufacturer must present substantive scientific evidence of a product's safety, efficacy and quality as required by the Food and Drugs Act and Regulations.
From: Health Canada webpage.
They have a database of products, but I suppose that you can always contact them if you have questions or concerns regarding a particular product on Canadian market.
Please note that it is very difficult (if not impossible) to directly compare concentrations in in an ointment (especially a lipid-based one) and in water or aqueous medium. The ultimate burden of testing the efficacy is on the manufacturer; the ultimate burden of ensuring that such tests have been conducted and gave sufficient results in on the regulatory agency responsible for a certain market.
- Sweetman SC (Ed), Martindale: The Complete Drug Reference. London: Pharmaceutical Press. Electronic version, (34th Edition ).
- Antiseptics and Disinfectants: Activity, Action, and Resistance Gerald McDonnell, A. Denver Russell, Clin Microbiol Rev. 1999 Jan; 12(1): 147–179.
- Protection of bacteria against toxicity of phenol by immobilization in calcium alginate Heribert Keweloh, Hermann-Josef Heipieper, Hans-Jürgen Rehm, Applied Microbiology and Biotechnology September II 1989, Volume 31, Issue 4, pp 383-389
- Health Canada official website